The invention relates to a method for the production of a dental prosthesis to be fastened to a tooth stump consisting of a three-dimensionally mapped and digitized positive model which comprises the following steps:                inputting the data of the digitized positive model;        machining the inner surface of the positive model with a preset offset value to form a gap between a future dental prosthesis and the tooth stump, and        calculating a program for machining a blank by means of a machine tool to produce the dental prosthesis.        
In particular, the invention relates to the field of producing basic structures for dental prostheses, in particular for dental crowns and/or bridges for fastening to prepared natural and/or artificial tooth stumps or the like.
A number of devices and methods for producing artificial dental bridges and crowns are known. Generally, after the dental preparation in which the teeth used for anchoring are prepared by grinding for receiving a crown or bridge or for which, e.g. a pin is implanted, an impression of the tooth stump, the surrounding area and jaw is made. This is usually done with silicone sealing compounds, but other materials are also known.
A so-called master model can be made from the impression by means of a plaster cast. This master model shows the situation in the patient's mouth positively. In this model, the dental technician with his handicraft skills fashions a model of the basic structure of the dental prosthesis from wax or from plastic which melts at a low temperature or hardens in a polymerizing manner (positive model). In this case, the dental technician can also take the counter occlusion of the other jaw into account by means of the master model in hand.
Traditionally, the model produced by the dental technician is embedded and melted in heat-resistant substances. The basic structure can be made by precision casting in conventional metal dental alloys in the mold thus produced.
For cosmetic reasons, a facing in ceramic or plastic is usually also made, at least in the area of the front teeth.
It is known from WO 99/47065 to completely digitalize the outer and inner surface of the structural model after a wax model has been formed. A model which inadequately reflects the situation in the patient's mouth is then mathematically completed with respect to the three-dimensional outer and inner surface. The result of the digitalization and a mathematical completion should represent a digital description of the complete surface of the basic structure of the prosthesis.
In order to be able to machine a blank consisting of porous ceramics with the data obtained from the digitalization of the wax model, it is described in detail in WO 99/47065 that the dimensions of the surface model of the digitized body are linearly enlarged in all spatial directions to compensate for the shrinkage of the blank during sintering. The enlargement factor f should thereby result from a specific function according to the ratio of the density of the blank and a basic bridge structure produced therefrom.
The control commands for a processing machine are to be generated from the data of the enlarged surface with which the enlarged basic bridge structure is to be carved out of the blank. In comparison to the mathematically enlarged surface of the basic bridge structure, no offset is provided, whereby the machined surface is to obtain exactly the same measurements vis-á-vis the digitized body after the sintering shrinkage. Furthermore, no subsequent processing should take place.
In addition, it is proposed in WO 99/47065 to use a blank for the production of the basic bridge structure in which a machine or visually readable identification code containing the enlargement factor f is affixed to the blank itself or to its packaging, to a tag or an enclosed packaging slip.
The wax model of the basic bridge structure should be produced positively on a plastic impression of the negative form of the prepared tooth stump produced from a silicone mass, whereby the tooth stumps should first be coated by hand with a spacer coating composition to later form a cement gap.
The digitalization is accomplished mechanically or optically. For this purpose, reference is made to methods for the digitalization in the mouth of a patient on a prepared tooth stump or to a model, said methods being known, for example, from U.S. Pat. No. 4,182,312 with respect to a mechanical digitalization and from EP 0 054 785 A1 with respect to an optical digitalization.
The fundamental disadvantage of the mechanical digitalization known from U.S. Pat. No. 4,182,312 is in the fixing of the mechanical scanning device to the patient since the scanning is to take place directly in the oral cavity of the patient. The secure handling of the device in the narrow oral cavity is equally problematic. A processing machine for producing dental prostheses should be controlled directly with the scanning of teeth and surrounding tissue as in a duplicating milling machine.
To this end, a probe having a transmission rod securely fixed to it must be moved by the dentist over the surfaces in the patient's mouth that are of interest. A complete detection of the surface requires very many scanning movements, which is very stressful for the patient due to the time required. Furthermore, the probe tips must be changed depending on the shape of the processing tool.
With the method described in EP 0 054 785 A1, an image recording head is to be inserted into a patient's mouth. This image recording head is to detect a three-dimensional image of a tooth cavity or the like. For this purpose, the image data is to be shown on a computer screen, so that a dentist can check to see whether the positioning of the image recording head enables a sufficiently accurate image. If necessary, the image recording head can be changed accordingly to a more favorable position.
When a proper position has been obtained, a three-dimensional image of the tooth cavity or the like should be formed spatially true to size—without further explanation. The appropriate data is then to be completed by interpolation and manual processing of the data set in the manner of a CAD construction, until a corresponding dental prosthesis body has been completely formed. The corresponding data should then be used to make a suitable blank in order to produce a suitable dental prosthesis directly from the image while avoiding the aforementioned skilled production steps.
The awkward manipulation with the camera in the patient's mouth was also found to be disadvantageous in practice with this method, in particular, it requires great discipline on the part of the patient.
Furthermore, as described in the aforementioned document, it is necessary to coat the tooth to be mapped with a powder to obtain defined reflection conditions, since the natural dental material has translucent properties. Due to the translucent properties, light could otherwise penetrate partially uncontrolled into the tooth stump to be measured and perhaps be reflected in deeper layers which would result in an inaccurate result. However, the coating with a reflection powder simultaneously increases the inaccuracy by the application of the powder which will inherently and, based on the restricted conditions in the patient's mouth, always be irregular in practice. The limited resolving power of the image recorder and the difficult lighting conditions in the mouth to be mapped are also disadvantageous.
A method for producing dental prosthesis parts is known from DE 196 42 247 C1 according to which a prepared tooth is digitized in order to then produce a dental prosthesis taking the digitized model teeth into consideration. To produce a dental prosthesis, according to WO 94/27523, a tooth is measured, a part of a tooth is prepared, a tooth impression or a copy of the tooth is made. The triangulation method is used for the measuring.
A powder-metallurgical production process for an accurately shaped dental prosthesis is known from EP 0 774 933 B 1. In this case, the three-dimensional optical or mechanical inputting of the prepared tooth takes place directly in the mouth or on a plaster model. The cement layer, by means of which the dental prosthesis is joined with the prepared tooth, is also taken into consideration when producing the dental prosthesis.
Independent of the type of production of the dental prosthesis, for aesthetic reasons, care is taken to make it as slender as possible. From experience, the dental model is also formed primarily from an aesthetic point of view to have sufficient space for the subsequent facing. As a result, the necessity for a sufficient mechanical construction is disregarded and the life of the dental prosthesis produced in accordance with it is adversely affected.